The present invention relates to a method and apparatus for recognizing meandering of a web, which are suitable for detection of a defective printed matter in a printing press, by comparing prestored reference image data with test image data obtained from a web on which a test object is printed.
In recent years, each pixel data (density data) of a printed matter serving as a reference is read and stored as reference image data by using an image processing means having an optical image pickup system, and then each pixel data of a test object is read as test image data by using the same image processing means as described above. The stored reference image data is compared with the test image data to automatically detect a defective printed matter.
The defective printed matter detecting apparatus for a printing press includes an image pickup means such as a CCD camera or a line sensor, a signal processor for processing an electrical signal as an output signal from the image pickup means, and a reference signal generating means for supplying a reference signal for processing to the signal processor. The level of pixel data (pixel position data) of the reference image data is compared with the level of the corresponding pixel data of the test image data to determine whether a difference between the levels of the reference and test image data falls within a predetermined range, thereby determining the defective or nondefective printed matter.
The printed matter serving as a reference for extracting the reference image data is a printed matter which is visually determined as a satisfactory printed matter selected from actual printed matters obtained in test printing at the start of printing.
According to the conventional defective printed matter detecting apparatus, when a rotary press is assumed, the corresponding pixel positions of the reference and test image data are slightly shifted from each other by meandering of a web on which the test object is printed, and this shift is determined as a defect.
More specifically, in the rotary press, the web slowly meanders during printing due to a slight difference in tension or the like. For this reason, although the corresponding pixel positions of the reference and test image data are slightly shifted from each other, the printed matter can be circulated as a satisfactory product unless this shift is large.
In the conventional defective printed matter detecting apparatus, since the reference image data at the start of printing is permanently used, the level difference between the reference and test image data is gradually increased with meandering. The resultant printed matter is determined as a defective printed matter although it can be circulated as a satisfactory product.